1. Field of the Invention
The present invention relates to a high-frequency filter, and more specifically, relates to a high-frequency filter including a plurality of LC parallel resonators.
2. Description of the Related Art
As a high-frequency filter of the related art, for example, a laminated band pass filter has been known that is described in International Publication No. 2007/119356. FIG. 10 is the exploded perspective view of the laminated band pass filter described in International Publication No. 2007/119356. FIG. 11 is the equivalent circuit diagram of the laminated band pass filter described in International Publication No. 2007/119356.
As illustrated in FIG. 10, a laminated band pass filter 500 is a laminated body including a ground electrode forming layer 401, a capacitor electrode forming layer 402, an input-output electrode forming layer 403, a line electrode forming layer 404, and an outer layer 405, and includes four LC parallel resonators and a between-input-and-output capacitor electrode 260.
Capacitor electrodes 411, 412, 413, and 414 in the capacitor electrode forming layer 402 face a ground electrode 409. As a result, capacitors C501 to C504 illustrated in FIG. 11 are configured. A via electrode 441 connects an input-output electrode 721 and the capacitor electrode 411 to each other. A via electrode 442 connects an input-output electrode 722 and the capacitor electrode 414 to each other.
As illustrated in FIG. 10, a via electrode 431 connects the capacitor electrode 411 and one end of a line electrode 616 to each other, and a via electrode 432 connects the other end of the line electrode 616 and the ground electrode 409 to each other. Accordingly, an inductor L501 illustrated in FIG. 11 is configured. A via electrode 433 connects the ground electrode 409 and one end of a line electrode 617 to each other, and a via electrode 434 connects the other end of the line electrode 617 and the capacitor electrode 412 to each other. Accordingly, an inductor L502 illustrated in FIG. 11 is configured. A via electrode 435 connects the ground electrode 409 and one end of a line electrode 618 to each other, and a via electrode 436 connects the other end of the line electrode 618 and the capacitor electrode 413 to each other. Accordingly, an inductor L503 illustrated in FIG. 11 is configured. A via electrode 437 connects the capacitor electrode 414 and one end of a line electrode 619 to each other, and a via electrode 438 connects the other end of the line electrode 619 and the ground electrode 409 to each other. Accordingly, an inductor L504 illustrated in FIG. 11 is configured.
As a result of including such individual electrodes as described above, the laminated band pass filter 500 includes a four-stage LC parallel resonance circuit and a coupling capacitance C514.
In addition, in the laminated band pass filter 500, so as to obtain a desired pass band characteristic, LC parallel resonators not adjacent to each other are capacitively coupled to each other. Here, the capacitive coupling between the LC parallel resonators not adjacent to each other is capacitive coupling between an LC parallel resonator including the inductor L501 and the capacitor C501 and an LC parallel resonator including the inductor L504 and the capacitor C504. In more detail, the between-input-and-output capacitor electrode 260 is provided in the input-output electrode forming layer 403, and faces the capacitor electrode 411 and the capacitor electrode 414. Accordingly, capacitance is generated between the between-input-and-output capacitor electrode 260 and the capacitor electrode 411, and capacitance occurs between the between-input-and-output capacitor electrode 260 and the capacitor electrode 414. As a result, the LC parallel resonator including the inductor L501 and the capacitor C501 and the LC parallel resonator including the inductor L504 and the capacitor C504 are capacitively coupled to each other. The coupling capacitance C514 in FIG. 11 indicates capacitance occurring between the LC parallel resonator including the inductor L501 and the capacitor C501 and the LC parallel resonator including the inductor L504 and the capacitor C504.
Incidentally, in the laminated band pass filter 500, so as to obtain the desired pass band characteristic, the coupling capacitance C514 is produced between the LC parallel resonator including the inductor L501 and the capacitor C501 and the LC parallel resonator including the inductor L504 and the capacitor C504, as described above. Therefore, the between-input-and-output capacitor electrode 260 is located near the capacitor electrodes 411 and 414. In addition, so as to produce the resonance capacitors C501 and C504 illustrated in FIG. 11, the capacitor electrodes 411 and 414 are located near the ground electrode 409. As a result, in the laminated band pass filter 500, the between-input-and-output capacitor electrode 260 is located near the ground electrode 409. Accordingly, in the laminated band pass filter 500, since unintended capacitive coupling occurs between the between-input-and-output capacitor electrode 260 and the ground electrode 409, it has been difficult to obtain the desired pass band characteristic.